1. Field of the Invention
The present invention relates to an optical amplifier used for an optical transmission system or an application for an optical measuring instrument. More particularly, the present invention relates to an optical amplifier which can prevent an optical surge from occurring when an output signal is interrupted for a moment.
In an optical transmission system, an optical amplifier is used for increasing a transmissible distance or reducing a system cost of a conventional 3R regeneration relay. Additionally, a demand for an optical amplifier which can generate high-power light to be used in a measuring application for design or evaluation of an optical transmission has been increased.
2. Description of the Related Art
In an optical transmission system or measuring application, many systems are used in which an optical amplifier using an erbium doped fiber (EDF) directly amplifies an optical signal. In such an EDF amplifier, a high-power light (optical surge) which greatly exceeds a preset optical output level is transitively generated during an instantaneous interruption of an input optical signal.
The optical surge can be defined as follows. It is assumed that an optical power of an output signal shown in a lower half of FIG. 1 is obtained as a result of amplification of an input signal having an optical power shown in an upper half of FIG. 1. The optical surge is defined by a ratio of a peak level to a normal output level when a high-power light having a level exceeding a normal level is generated. EQU optical surge level (dB)=10 log(p2/p1)
where P2 is a peak level (W) of the output optical surge, and P1 is a normal optical output level (W).
It should be noted that .DELTA.tr shown in FIG. 1 is a period of time on an order of micro seconds (.mu.s).
FIG. 2 is a conceptual illustration of generation of an optical surge.
FIG. 2-(a) indicates a state where both a pumping light and a signal light are incident on an optical amplifier medium (for example, EDF). EQU .DELTA.E=E2-E1=h.nu.
where .DELTA.E is an energy gap, E2 is an excitation level, E1 is a ground level or a low energy level, h is Plank's constant, and .nu. is a frequency of an output light. In this state, a ratio of density of optically amplifying media (Er ions) in each of the excited level (E2) and the ground level (E2) is constant, and the excited level and the ground level are balanced (A density N2 in the excited level and a density N1 in the ground level are equal to each other.)
FIG. 2-(b) indicates a state where the signal light input to the optically amplifying media is stopped or interrupted but the pumping light is continuously incident on the optically amplifying media. In this state, the ratio of density of the optically amplifying media in the excited state is increased (N2&gt;&gt;N1), and an excitation rate (population inversion ratio) is at a high level. Accordingly, a spontaneous emission light which satisfies the relationship .DELTA.E=E2-E1=h.nu. is emitted.
FIG. 2-(c) indicates a state where the input signal light is returned from the interrupted state to the normal state. In this state, the optical amplifier media at the excited level emits energy to the input signal light more than that of the state (a) so that the optical amplifier medium rapidly returns to the state (a) where the ratio of density of the optical amplifier medium at either the excited level or the ground level is balanced. As a result, the signal output after the return has an optical surge having a level exceeding the normal output level.
FIG. 3 shows a relationship between an amount of the optical surge and the instantaneous interruption time in a case where the pumping light is stopped when the interruption of the pumping light occurs. As shown in FIG. 3, the amount of the optical surge decreases when the instantaneous interruption time exceeds about 1 ms.
A relationship between the optical surge and the instantaneous interruption time when the pumping light is not stopped is shown in FIG. 4. The graph shown in FIG. 4 includes an optical surge ungenerated time area A and an optical surge generated time area B. In the optical surge ungenerated time area A, an optical surge is not generated since the interruption time is not so short such that a rare earth doped fiber used as an optical amplifying medium cannot respond.
As a means for preventing an occurrence of such an optical surge, two measures have been taken conventionally. One of them is shown in FIG. 5. In the conventional system shown in FIG. 5, an amount of optical surge is suppressed by stopping the pumping light by detecting an interruption of the input signal light. The suppression of the amount of the optical surge can be achieved by speeding up the stopping of the pumping light. That is, the optical surge in the output signal light can be suppressed by decreasing accumulation of energy at a minimum by speeding up the stopping of the pumping light.
In the system shown in FIG. 5, the amount of optical surge is suppressed by stopping the pumping light by detecting the input signal light. In this system, an erbium dopes optical fiber EDF is used as an optical amplifying medium. The input signal light is input to an input terminal 1, and is synthesized with the pumping light by a wave length division multiplexer WDM. The synthesized light is incident on the erbium doped optical fiber FDF, and an amplified output signal light is obtained at an output terminal 2.
Additionally, in the system shown in FIG. 5, the input signal light input from the terminal 1 is monitored by a path of a beam splitter BS and a photo diode PD so as to stop the pumping light by decreasing a reference voltage (ALC-ref) of an ALC (automatic level control) circuit when the input signal light is decreased to a threshold value (Vref).
FIG. 6 is a circuit diagram of an example of the ALC circuit. A relationship between the optical surge and the instantaneous interruption time of the example shown in FIG. 6 corresponds to that shown in FIG. 3. Different from the graph shown in FIG. 4, the graph shown in FIG. 3 includes an optical surge ungenerated time area C. This area is generated due to stopping of the pumping light. That is, as shown in FIG. 2-(b), even if a density of carriers at an excitation level is high at an initial state, the density at the excited level decreases as radiation light is gradually emitted while no carrier is raised to the excited level due to the stopping of the pumping light. As a result, the density of carriers at the excited level is decreased and, thereby, the optical surge is not generated in the area C when the input light returns. The start time of this time area is substantially equal to an average life time of the carrier at the excited level in the rare earth doped optical fiber, which is an optical amplifying medium, when the time is measured from the time when the pumping light is stopped.
The amount of optical surge in the circuit shown in FIG. 5 is dependent on 1) an output pumping light entering after the interruption of the input signal light, 2) an increasing speed of the input signal light which returns after the interruption and 3) the interruption time of the input signal light. Among these factors, the factor 1) is able to be handled in the optical amplifier by increasing the speed of stopping the pumping light, whereas the factors 2) and 3) cannot be handled by the optical amplifier alone since the factors 2) and 3) are external factors for the optical amplifier. Accordingly, an optical surge may always be generated depending on the condition of the increasing speed at the time for returning the input signal light after an interruption and an interruption time of the input signal light.
FIG. 7 shows an example of a system which improves on the system shown in FIG. 5. In the system shown in FIG. 7, an optical transmission delay fiber TDF for delaying transmission is connected subsequent to the optical amplifying medium EDF, and an optical surge which has already been generated is suppressed by using a variable attenuator element ATT. A part of the amplified signal is split by a beam splitter BS2, and is monitored by a photo diode PD2 so that, when an optical surge is generated, the optical surge is prevented from being output from the output terminal 2 by increasing a degree of attenuation while the optical surge is delayed by the transmission delay optical fiber TDF.
In this technique, 1) the transmission delay optical fiber is needed, and 2) the variable attenuator element must be connected subsequent to the optical amplifying medium EDF. Since the variable attenuator element at the present time generates a transmission loss (1 to 2 dB), the loss must be compensated so as to obtain the same output.
Accordingly, the above-mentioned improved technique has drawbacks in that:
1) the need for the transmission delay optical fiber is a problem for downsizing the optical amplifier; and PA1 2) the connection of the loss generating factor is a problem for obtaining a high output. PA1 an input signal light interruption detecting circuit detecting an interruption of the input signal light; and PA1 a variable attenuator element attenuating the input signal light, the variable attenuator element provided on a signal light input side of the optical amplifying medium, PA1 wherein the variable attenuator element is controlled based on an output of the input signal light interruption detecting circuit so that a degree of attenuation of the variable attenuator element is increased when the input signal light is interrupted and the degree of attenuation is gradually decreased when the input signal light returns. PA1 an input signal light interruption detecting circuit detecting an interruption of the input signal light; PA1 a timer for measuring a time after the input signal light is interrupted upon receipt of an output of the input signal light interruption detecting circuit; and PA1 a variable attenuator element attenuating the input signal light, the variable attenuator element provided on a signal light input side of the optical amplifying medium, PA1 wherein the variable attenuator element is controlled based on the output of the input signal light interruption detecting circuit so that a degree of attenuation of the variable attenuator element is increased when a predetermined time is elapsed after the input signal light is interrupted and the degree of attenuation is gradually decreased when the input signal light returns. PA1 an input signal light interruption detecting circuit detecting an interruption of the input signal light; PA1 a timer for measuring a time after the input signal light is interrupted upon receipt of an output of the input signal light interruption detecting circuit; and PA1 a variable attenuator element attenuating the input signal light, the variable attenuator element provided on a signal light input side of the optical amplifying medium, PA1 wherein the variable attenuator element is controlled based on an output of the timer so that a degree of attenuation of the variable attenuator element is increased when the input signal light is interrupted and the degree of attenuation is decreased at a time when an optical surge is no longer generated after a period during which an optical surge is generated is passed. PA1 an input signal light interruption detecting circuit detecting an interruption of the input signal light; PA1 a timer for measuring a time after the input signal light is interrupted upon receipt of an output of the input signal light interruption detecting circuit; and PA1 a variable attenuator element attenuating the input signal light, the variable attenuator element provided on a signal light input side of the optical amplifying medium, PA1 wherein the variable attenuator element is controlled based on an output of the timer so that a degree of attenuation of the variable attenuator element is increased when a predetermined time is elapsed after the input signal light is interrupted and the degree of attenuation is decreased at a time when the optical surge is no longer generated.